This invention relates to processes for producing optically anisotropic carbonaceous pitch which is suitable for the production of carbon materials containing carbon fibers having a high tensile strength and high elastic modulus and other carbon materials. More particularly, the present invention relates to optically anisotropic carbonaceous pitch having a low softening point which is substantially homogeneous and which can be obtained by thermal decomposition polycondensation or other treatments of a liquid hydrocarbon mixture having a specific composition and structure as the starting material for the production of optically anisotropic carbonaceous pitch suitable for the production of carbon fibers and other molding carbon materials to be used for a composite material having a light weight, high strength and high elastic modulus, and relates also to processes for producing such carbonaceous pitch.
As energy saving and resource saving have become a need of the times, a keen demand has arisen for an economical high-performance carbon fiber as the raw material of a composite material having a light weight, high strength and high elastic modulus that is necessary for airplanes, cars and the like, or for a molding carbon material having a high strength and high density that can be adapted for a wide range of applications by pressure molding. The present invention provides a process for producing optically anisotropic carbonaceous pitch which has a low softening point, is homogeneous and excellent in the molecular orientation, is suitable for the production of high-performance carbon fibers and molding carbon materials such as those described above and can be easily molded, e.g., by melt-spinning.
As disclosed in Japanese Patent Application No. 162972/1980 filed previously by the inventors of the present invention, the inventors have examined various optically anisotropic pitch compositions suitable for producing high-performance carbon fibers, and have found that the optically anisotropic pitch has a good molecular orientation in which a laminate layer structure of condensed polycyclic aromatic groups has developed but various pitches exist as a mixture in practice. The inventors have also found that among these pitches, one that is suitable for the production of a homogeneous carbon fiber having a low softening point has a specific chemical structure and composition. That is to say, in the optically anisotropic pitches, the composition, structure and molecular weight of component O, i.e., an n-heptane soluble component, and component A, i.e., an n-heptane insoluble and benzene soluble component, have been found extremely important. Speaking in further detail, the inventors of the present invention have found that a pitch composition containing specific amounts of components O and A can exist as the optically anisotropic pitch and it is an essential condition for an optically anisotropic pitch composition to properly adjust the balance of the components in order to practically produce a high-performance carbon material.
The inventors have found further that an optically anisotropic pitch for producing carbon materials having higher performance can be provided by stipulating a quinoline soluble component (hereinafter referred to as "component B") and a quinoline insoluble component (hereinafter referred to as "component C") as the remaining benzene insolubles other than the abovementioned components O and A.
The inventors have further examined in detail the individual characteristics of each component and the relation between the content of each component having such characteristics and the properties, homogeneity and molecular orientation of the pitch as a whole, and have found that it is important that each component should be contained in a specified range of contents and should have specified range of properties. The inventors have thus found that the constituents of an optically anisotropic pitch which has high molecular orientation, homogeneity and a low softening point necessary for the production of a high-performance carbon fiber and can be melt-spun stably at a low temperature must have properties, such as the C/H atomic ratio, fa, number average molecular weight, maximum molecular weight (molecular weight at a point integrated by 99% from the low molecular weight side) and minimum molecular weight (molecular weight at a point integrated by 99% from the high molecular weight side) as specified within the following ranges, respectively.
The component O has the C/H atomic ratio of at least about 1.3, fa of at least about 0.80, the number-average molecular weight of up to about 1,000 and the minimum molecular weight of at least about 150. Preferably, the C/H atomic ratio is about 1.3 to 1.6, fa is about 0.80 to about 0.95, the number-average molecular weight is about 250 to about 700 and the minimum molecular weight is at least about 150.
The component A has the C/H atomic ratio of at least about 1.4, fa of at least about 0.80, the number-average molecular weight of up to about 2,000 and the maximum molecular weight of up to about 10,000. Preferably, the C/H atomic ratio is about 1.4 to about 1.7, fa is about 0.80 to about 0.95, the number-average molecular weight is about 400 to about 1,000, and the maximum molecular weight is up to about 5,000.
A suitable content of each component is about 2% to about 20% (by weight) for component O and is about 15% to about 45% (by weight) for component A. The optimal range is about 5% to about 15% (by weight) for component O and about 15% to about 35% (by weight) for component A.
If the C/H atomic ratio and fa of component O are below the abovementioned range and the content of component O is above the abovementioned range, the resulting pitch is likely to be heterogeneous as a whole and to contain a considerably great proportion of isotropic portions. If the average molecular weight is greater than 700 or if the content is below the abovementioned range, pitch having a low softening point can not be obtained. If the C/H atomic ratio or fa of component A is below the abovementioned range and if the number-average molecular weight is below the abovementioned range or the content exceeds the abovementioned range, the pitch is likely to become heterogeneous in which isotropic and anisotropic portions are mixed with each other. If the number-average molecular weight or the maximum molecular weight exceeds the abovementioned range or if the proportion of component A is below the abovementioned range, the resulting pitch will not have a low softening point, though it is homogeneous and optically anisotropic.
The inventors have furthered their studies and have found that components O and A are those which are entrapped in the laminate structure inside the optically anisotropic pitch, performs a solvent-like or plasticizer-like action, participate primarily in the meltability and fluidity of the pitch or are difficult by themselves to manifest the laminate structure and to exhibit the optical anisotropy, but when benzene insoluble components B and C, that are residual components, can not be melted by themselves and can easily form the laminate, are contained within the specified ranges in a well-balanced proportion with respect to components O and A and if the chemical structure, characteristics and molecular weight of each component are within the specified range, optically anisotropic pitch necessary for producing a high-performance carbon fiber having a low softening point can be obtained.
In other words, it has been found that an optically anisotropic pitch containing about 2 wt% to about 20 wt% of component O, about 15 wt% to about 45 wt% of component A, about 5 wt% to about 40 wt% of component B (a benzene insoluble, quinoline soluble component) and about 20 wt% to about 70 wt% of component C (a benzene and quinoline insoluble component) and having a content of its optically anisotropic phase of at least about 90% by volume and a softening point of up to about 320.degree. C. can provide further stabilized high-performance carbon fiber.
As for the abovementioned components B and C, as the constituents of an optically anisotropic pitch that can be melt-spun stably at a low temperature, they have the C/H atomic ratio, fa, number-average molecular weight, and maximum molecular weight (molecular weight at a point integrated by 99% from the low molecular weight side) within the specified ranges, respectively, as will be described below.
The component B (a benzene insoluble, quinoline soluble component) has the C/H atomic ratio of at least about 1.5, fa of at least about 0.80, the number-average molecular weight of up to about 2,000 and the maximum molecular weight of up to about 10,000. Preferably, the C/H atomic ratio is about 1.5 to about 1.9, fa is about 0.80 to about 0.95 and the number-average molecular weight is about 800 to about 2,000. The component C (a benzene and quinonline insoluble component) has the C/H atomic ratio of up to about 2.3, fa of at least about 0.85, the estimated number-average molecular weight of up to about 3,000 and the maximum molecular weight of up to 30,000. Preferably, the C/H atomic ratio is about 1.8 to about 2.3, fa is about 0.85 to about 0.95 and the number-average molecular weight of about 1,500 to about 3,000.
The contents of these components are as follows: content B is about 5 wt% to about 55 wt%, preferably about 5 wt% to about 40 wt% and content C is about 20 wt% to about 70 wt% preferably about 25 wt% to about 65 wt%.
The inventors of the present invention have carried out intensive studies on optically anisotropic carbonaceous pitches having a specific composition and characteristics of the abovementioned components O, A, B and C and have found that among these optically anisotropic carbonaceous pitches, those having extremely excellent characteristics contain the optically anisotropic phase within the range of 80% to 100%, have a softening point in the range of 230.degree. C. to 320.degree. C., the number-average molecular weight in the range of about 900 to about 1,200, contain molecules having the molecular weight of up to 600 in the range of 30 mol% to 60 mol%, molecules having the molecular weight of at least 1,500 in the range of 15 mol% to 35 mol% and molecules having the molecular weight of 600 to 1,500 in the range of 20 mol% to 50 mol% and have the maximum molecular weight of up to 30,000.
The optically anisotropic carbonaceous pitch in accordance with the present invention has a large content of the optically anisotropic phase, is homogeneous and has a sufficiently low softening point as well as good fludity and moldability of pitch.
Various methods have been proposed in the past to produce an optically anisotropic carbonaceous pitch required for the production of a high-performance carbon fiber. All of these methods have however failed to provide optically anisotropic carbonaceous pitch suitable for the production of a carbon material having high strength and high elastic modulus, containing components O and A having the abovementioned specific composition, structure and molecular weight and further components B and C and having a specific molecular weight distribution. Moreover, these conventional methods involve various drawbacks such as listed below.
(1) The starting materials are not easily available industrially. PA1 (2) The production calls for the reaction for an extended period of time or for the complicated process. Hence, the process costs high. PA1 (3) When the content of the optically anisotropic phase is brought close to 100%, the softening point increases and spinning becomes difficult. If the softening point is kept low, on the other hand, the pitch becomes heterogeneous and spinning becomes difficult. PA1 (1) An optically anisotropic carbonaceous pitch consisting of a substantially homogeneous, optically anisotropic phase and having a low softening point (e.g. 260.degree. C.) can be produced within a short period of time (e.g. 3 hours for the overall reaction) without calling for complicated and costly procedures such as high temperature filtration of unmolten matters, solvent extraction, removal of a catalyst, and so forth. Accordingly, a low optimal spinning temperature (the highest temperature suited for melting, fluidizing and transferring the pitch, and excluding gas bubbles inside a melt-spinning machine) can be set in the range of from about 290.degree. to 370.degree. C. and preferably from 300.degree. C. to 360.degree. C. in producing carbon fibers. PA1 (2) The optically anisotropic carbonaceous pitch produced in accordance with the process of the present invention is excellent in homogeneity and makes it possible to continuously spin a fiber of a substantially uniform thickness having a flat surface at a temperature by far lower than about 400.degree. C. and hence spinnability (in the aspects of frequency of yarn breakage, yarn thickness, yarn variance) is excellent. Since degradation does not occur during spinning, the quality of the carbon fiber as the product is stable. PA1 (3) Decomposition gases and unmolten matters are not substantially formed during spinning so that high speed spinning is feasible, the spun pitch fiber has fewer defects and the strength of the carbon fiber becomes high. PA1 (4) Since carbon fiber can be produced by spinning the optically anisotropic pitch which is substantially in the form of liquid crystal as a whole, the orientation of the graphite structure well develops in the direction of the fiber axis and a carbon fiber having high elastic modulus can be obtained.
More particularly, the method disclosed in Japanese Patent Publication No. 8634/1974 either uses chrysene, anthracene, tetrabenzophenazine or the like that can not be obtained economically in large quantities, or requires complicated production processes such as distilling the tar of crude oil cracked at high temperature and thereafter filtrating unmolten matters at high temperature. Moreover, the spinning temperature of as high as 420.degree. C. to 440.degree. C. is required in this prior art. The method disclosed in Japanese Patent Laid-Open No. 118028/1975 relates to a method of obtaining heavy oils from the tar of crude oil cracked at high temperature as the starting material, by heating with agitation, but the reaction for an extended period of time and filtration of unmolten matters in the pitch at high temperature are necessary in order to obtain a pitch having a low softening point. Japanese Patent Publication No. 7533/1978 discloses a method which polycondenses petroleum tar and pitch by use of a Lewis acid type catalyst such as aluminum chloride. Since removal of the catalyst and heat-treatments before and after the removal of the catalyst are necessary, the method is complicated and its operation cost is high. In heat-polymerizing an optically anisotropic pitch as the starting material, the method of Japanese Patent Laid-Open No. 89635/1975 carries out the reaction under the reduced pressure or while bubbling an inert gas into the liquid phase until the optically anisotropic phase content reaches 40% to 90%. The resulting pitch contains quinoline insolubles and pyridine insolubles in an amount equal to that of the optically anisotropic phase. Japanese Patent Laid-Open No. 55625/1974 discloses an optically anisotropic carbonaceous pitch whose content of the optically anisotropic phase is just 100%, but the softening point as well as the spinning temperature are considerably high. As the starting material, the prior art reference discloses nothing but the use of certain commercially available petroleum pitches. If the pitch is produced in accordance with this process using many kinds of starting materials such as coal tar, distillation residue of petroleum and the like, the molecular weight becomes so great that spinning becomes infeasible due to the formation of unmolten matters or the rise of the softening point and spinning temperature. Thus, none of the prior art stipulate the composition or structure of the starting materials and they have practically failed to stably provide a carbonaceous pitch having a predetermined high quality.
To solve these problems of the prior art, the inventors of the present invention provided a novel technique, disclosed in the specification of the prior Japanese Patent Application No. 11124/1981. Namely, if an oily matter having a specific molecular weight and aromatic carbon fraction, fa, is selected from an oily matter containing a principal component with a boiling point in the range of 250.degree. C. to 540.degree. C., a homogeneous, optically anisotropic pitch having a low softening point can be obtained by subjecting the oily matter to thermal decomposition polycondensation and other necessary treatment. The present invention is completed by further developing this technique and uses, as the starting material, a heavier matter or a so-called "tar-like" material which contains at least a component having a boiling point of 540.degree. C. or above as a principal component and preferably also contains a component having boiling points in the range of 360.degree. C. to 540.degree. C. The inventors have found that if a tar-like matter containing non-saturated components (which will be described later in further detail) having specified molecular weight and fa are employed, a homogeneous, optically anisotropic pitch having a low softening point can be obtained stably with a high yield. Thus, the present invention is completed.
The boiling point range of the principal components of 360.degree. C. or above and 540.degree. C. or above as mentioned before almost corresponds to that of the distillation residue of heavy oils obtained by the distillation operation that can be generally carried out easily on a large scale by use of a distillation apparatus used generally in the petroleum or coal industry. Moreover, it corresponds to the boiling point range of effective components that can be thermally converted into pitch with a high yield.
Among the conventional technique, those disclosed in Japanese Patent Laid-Open Nos. 160427/1979, 58287/1980, 144087/1980, 2388/1981 and 57881/1981 are methods to concentrate those components which can easily form an optically anisotropic phase by effecting solvent extraction of an optically isotropic pitch or pitch containing a small amount of an optically anisotropic pitch. However, it is not clear what starting materials is to be used for these methods. There are a large variety of optically isotropic pitches or pitches containing an optically anisotropic phase in existence. In the case of these pitches, too, the characteristics of the pitches depend upon the molecular weight distribution and contents of aromatic groups of the starting heavy oils and desired pitches can not be obtained with reproducibility, though the pitches can be obtained at times.
As disclosed in Japanese Patent Laid-Open No. 57881/1981, optically anisotropic pitches produced in accordance with these methods have generally a high softening point of 320.degree. C. or more in most cases, though the molecular weight distribution is relatively narrow. Accordingly, the optimal temperature for spinning the pitches is mostly close to 380.degree. C. or above in which the thermal decomposition polycondensation reaction of the pitches can occur. In mass-producing pitch fibers on an industrial scale, therefore, problems are likely to occur in the operation and quality management. For, though solvent extraction of the optically anisotropic pitch to adjust its molecular weight distribution and distribution of the aromatic structure makes it possible to prepare an optically anisotropic pitch having a reduced content of high molecular weight components, low molecular weight components are likely to be removed excessively by the solvent so that components contributing to the fluidity of the resulting optically anisotropic phase decrease, eventually increasing the softening point and spinning temperature of the resulting optically anisotropic pitch.
In producing optically anisotropic pitch only by thermal decomposition polycondensation without resort to solvent extraction in accordance with the conventional method such as one disclosed in Japanese Patent Publication No. 1810/1979, the molecular weight and structural characteristics of the starting materials are not known. It is assumed that since the thermal decomposition polycondensation is carried out while removal of volatile matters is being strongly promoted by bubbling a large quantity of an inert gas, the content of low molecular weight aromatic hydrocarbons decreases in the resulting optically anisotropic phase, the optically anisotropic phase becomes essentially quinoline or pyridine insoluble and its softening point and spinning temperature become relatively high.
In contrast, when the present invention is employed, especially when the starting material of the present invention having the molecular weight distribution and characteristics of an aromatic structure within the specified ranges are employed, all the abovementioned problems with the prior art can be eliminated and hence, a peculiar optically anisotropic pitch capable of providing carbon materials for carbon fibers and graphite fibers of a higher quality can be obtained stably with a high yield and at low cost.